1. Field of the Invention
The present invention relates to the field of data communication and, more particularly, to an equalizer circuit, communication system and method for receiving a communication signal sent across a transmission channel and for accurately recovering the communication signal having a wide variance of communication signal launch amplitude, wide performance fluctuations in the transmitter or equalizer, or the length/attenuation of the transmission channel.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
A data communication system typically involves a transmitter and a receiver connected by a transmission path. The transmitter may transmit modulated communication signals or communication signals that are coded. Popular types of modulation include pulse modulation, frequency modulation (e.g., frequency-shift keying), phase modulation (e.g., phase-shift keying), amplitude modulation (e.g., quadrature amplitude modulation), and numerous others. Popular types of coding include block codes and convolutional codes that encode digital information into a communication signal that can be sent across rather long transmission paths.
A transmission path can either be a wireless path or, alternatively, a “wired” path. Acceptable forms of a wired transmission path include electrical conductors, such as coaxial cables, unshielded twisted pair cables, shielded twisted pair cables, or fiber optic cables. A typical transmission medium exhibits a low pass characteristic. The low pass characteristic allows the transmission path to transmit low frequency components of a communication signal more readily than high frequency components of that signal. Thus, a transmission path has a frequency response that is not flat and, instead, has certain time dispersive functionality.
A modulated and/or coded communication signal sent across the transmission path generally consists of symbols. The symbols are often packed close to one another in a time domain. When the symbols are passed through the transmission line having a low-pass characteristic, the symbols can oftentimes spill over into each other causing what is known as “inter-symbol interference” or ISI.
To compensate for the low pass characteristic of the transmission path and to offset any occurrence of ISI, communication signals are typically passed through an equalizer circuit at the receiver end of the communication system. The equalizer is chosen as having a high-pass characteristic and, ideally, has a frequency response that is exactly inverse to that of the low-pass transmission path.
In most practical applications, the low-pass characteristic of the transmission path can change depending on the media used, as well as the length of the transmission path. As the transmission path changes, it is beneficial that the equalizer also change to avoid under-compensation (undershoot) or over-compensation (overshoot). There are several mechanisms used to compensate for transmission line changes. For example, the equalizer can be designed with a particular transmission line in mind. Knowing the transmission line characteristic will thereby allow the manufacturer to pair a more appropriate equalizer to that line. Alternatively, the frequency response of a transmission line can be measured during operation, and the equalizer high pass characteristic can be changed or adapted to any such measured change.
One problem with the first approach is the implausibility in having to redesign and substitute different equalizers whenever the transmission path changes. Moreover, it is often difficult to maintain accurate matching between the transmission path and the equalizer if either one of those components should change with changes in fabrication processes, or changes in temperature or voltage during operation. If the second approach is to be used, most conventional adaptive equalizers have a limited range to which they can adapt. For example, an adaptive equalizer cannot provide a matching inverse frequency response if the transmission path extends beyond a maximum range. Alternatively, the same can be said if the transmission path is shorter than a minimum range. In addition, adaptive equalizers are oftentimes limited to communication signals that fall within an amplitude range. If a communication signal amplitude extends outside that range, then exact equalization may not be possible.
Therefore, a need exists in having an equalizer that can produce a more exact inverse frequency response to match the transmission path, regardless of the length of that path or the amplitude of the communication signal transferred across that path. The desired equalizer, when placed in a communication system, can adapt to any of the various characteristics of the transmission path, and can also apply the appropriate gain and rolloff characteristics in order to minimize any high frequency noise, jitter, and distortion of the symbols that cause the loss or incorrect communication of recovered information.